Technical Abstract:
Chronic stress in chickens is a serious welfare concern in the egg industry. To address genetic differences in stress susceptibility, three strains of White Leghorn hens were randomly housed in a low (4 birds/cage, 213 cm2/bird) or high (10 birds/cage, 175 cm2/bird) group size and density cage system from 17 wks of age resulting in six treatments groups (n=12 per treatment), i.e., HPHS10, HPHS4, LPLS10, LPLS4, DXL10, and DXL4. These strains included a high producing and high survivability strain (HPHS), a low producing and low survivability strain (LPLS), and a commercial strain Dekalb XL (DXL). At 30, 45, and 60 wks of age, blood samples were taken from 12 birds per treatment. Plasma catecholamines (dopamine (DA), adrenaline (A), and noradrenaline (NA)) along with serotonin (5-HT) and tryptophan levels were quantified using High Performance Liquid Chromatography. At 30 wks of age, there were no significant treatment differences in catecholamines levels. Within strain, 5-HT levels were elevated in LPLS4 compared to LPLS10 (P<0.05). Between strains, only tryptophan levels tended to be higher in HPHS4 than in LPLS4 (P<0.01). By 45 wks, however, both A and NA levels were higher in LPLS10 relative to LPLS4 (P<0.05) whereas DA levels tended to be lower in LPLS10 (P<0.10). Serotonin levels were higher in LPLS4 than in LPLS10 (P<0.05) and DXL4, respectively (P<0.05). Tryptophan levels were present at greater levels in LPLS4 than HPHS4 (P<0.10 tendency) at 45 wks. At 60 wks, DA levels were elevated in HPHS10 compared to DXL10 and LPLS10 (P<0.05), but levels in DXL10 and LPLS 10 were the same. Serotonin levels were higher in LPLS4 relative to DXL4 (P<0.10) and HPHS4 (P<0.05). There were no treatment differences in the levels of A and NA at 60 wks. These results indicate that chronic social stress-induced different changes of the neurotransmitter system among the present chicken lines. These differences may associate with its line-unique coping ability to domestic environments and survivability.